Why is my rendered animation showing a hexadecagon border around my image?
Here's a plane:

Here's the same plane with a subsurf modifier:

Now the object has more vertices instead of the original 4.
The subdivision modifier creates new &quot;virtual&quot; geometry, by interpolating the existing edges and faces.

The Subdivision Surface modifier (often shorten to “Subdiv”) is used to split the faces of a mesh into smaller faces, giving it a smooth appearance. It enables you to create complex smooth surfaces while modeling simple, low-vertex meshes. It avoids the need to save and maintain huge amounts of data, and gives a smooth “organic” look to the object.

If you use (default) Catmul-clark subdivision the object will be rounded.
To keep the plane like a parallelogram, use Simple subdivision: